Research wins major NASA grant

ANITA project expands on earlier missions

Locations of ultra-high energy cosmic-ray events detected in Antarctica by the ANITA experiment.

The ANITA payload shortly after launch. Inset: The inflated balloon and ANITA payload at 120,000 ft.

NASA has awarded the University of Hawai‘i at Mānoa and five U.S. institutions a $4.6 million grant for the Antarctic Impulsive Transient Antenna (ANITA) experiment. UH Mānoa, the lead institution, will receive $1.4 million of the total amount.

The ANITA project is designed to view the Antarctic ice sheet over a wide area using a sophisticated array of antennas. They “listen” for sharp bursts of radio waves emitted by cosmic high energy neutrinos as they interact deep within the ice sheet, producing what amounts to a mini-bolt of lightning within the ice sheet. The payload of 40 antennas is carried around the Antarctic ice sheet by a stadium-sized balloon circling the continent at an altitude of 120,000 feet—four times as high as a passenger airplane travels. Radio bursts from within the ice can travel through the radio-transparent ice, emerging to be seen from above by ANITA.

Peter Gorham, a UH Mānoa professor who serves as principal investigator, is one of approximately 45 researchers who’ve been working on the ANITA experiment since 2003. In a previous flight in 2007, the team detected a handful of radio bursts that didn’t fit the profile for neutrinos, but could not be tied to any other source either, creating a puzzle for the researchers. Later searches through that data yielded a total of 16 such events. The team members then realized that they were observing radio signals from ultra-high-energy cosmic-rays—protons or atomic nuclei rather than neutrinos. These cosmic-rays were making radio bursts in the Earth’s atmosphere, which then reflected off the ice surface, to be seen by ANITA as in a huge mirror.

“These cosmic-ray events were a real surprise for us,” says Gorham. “It is a completely new way to detect these rare particles, and it may turn out that observations by a balloon or spacecraft payload are the most powerful way to see the highest energy particles in the universe.” The paper describing this discovery was chosen as the cover article in the October 8 edition of Physical Review Letters, one of the premier scientific journals in physics.

Neutrinos are subatomic particles, the smallest building blocks of all matter, which interact so rarely with other matter that one could pass untouched through a wall of lead stretching from the Earth to the moon. At the highest energies where ANITA observes them, neutrinos and cosmic-ray particles are thought to be closely related, both arising from some of the most exotic sources in the universe, such as massive black holes, quasars, and gamma-ray bursts from exploding stars.

The planned next flight of ANITA will focus on gaining much more knowledge about both high energy neutrinos, which have yet to be conclusively detected, and the newly detected cosmic rays. “We saw two possible neutrinos in a previous flight,” says Gorham. “In our next flight, we hope to see a more convincing set of neutrinos, and several hundred cosmic-ray events.” The next ANITA flight is expected to take place about three years from now.

In addition to NASA’s Jet Propulsion Laboratory, other universities participating in the ANITA project led by UH Mānoa include University of California, Los Angeles; Ohio State University; University of Delaware; University of Kansas; Washington University, St. Louis; National Taiwan University and University College London.